The invention relates generally to offshore marine seismic prospecting and, more particularly, to systems and methods for controlling the spread of towed seismic streamers.
In the search for hydrocarbon deposits beneath the ocean floor, a survey vessel 10, as shown in FIG. 1, tows one or more seismic sources (not shown) and one or more streamer cables S1-S4 instrumented with hydrophones and other sensors. In multiple-streamer systems, the streamers are towed underwater behind the survey vessel in a generally parallel arrangement. The tail end of each streamer is tethered to a tail buoy 11 that marks its position. The seismic source periodically emits a seismic wave that propagates into the ocean floor and reflects off geologic formations. The reflected seismic waves are received by the hydrophones in the streamers. The hydrophone data is collected and later processed to produce a map of the earth's crust in the survey area.
The quality of the survey depends on, among other things, knowledge of the precise position of each hydrophone. Position sensors, such as heading sensors and acoustic ranging devices 12, located along the lengths of the streamers are used to determine the shapes of the streamers and their relative separations. The acoustic ranging devices are typically acoustic transceivers operating on a range of channels over which they transmit and receive acoustic ranging signals to and from one another to produce accurate ranges 14 between their locations on the streamers. The many ranges—only a few are shown in FIG. 1—and streamer heading data from the many heading sensors are used to compute a network solution that defines the shapes of the individual streamers and their relative positions. When one point on the streamer array is tied to a geodetic reference, such as provided by a GPS receiver, the absolute position of each hydrophone can be determined.
Positioning devices, such as depth-keeping birds and lateral steering devices 16 located at nodes along the lengths of the streamers, are used to control the depths of the streamers and their separations from each other. The positioning devices could be equipped with acoustic ranging devices to range with other positioning devices so equipped and with dedicated acoustic ranging devices. Precise positioning of the streamers is important during online survey passes to produce a high-quality map. Cross currents, however, cause the streamers to deviate from straight lines parallel to the towing vessel's course. Instead, the streamers may angle straight from their tow points or assume a curved shape with their tail ends tailing away from the straight lines. This feathering of the streamers is often undesirable in online survey passes. Precise positioning is also important during turns between online survey runs to reduce the time of the turn without entangling the streamers.
In conventional streamer positioning systems, one of the streamers, outermost port streamer S1 in this example, is used as a reference streamer. A shipboard controller 18 collects the position sensor data and computes the network solution representing the shapes of the streamers and their separations from each other. From the network solution and the target separations of corresponding steering-device nodes on the streamers referenced directly or indirectly to points on the reference streamer, the shipboard controller derives steering commands for each lateral steering device. The steering commands are transmitted to the steering devices to adjust their control planes, or fins, to drive the streamers laterally, as indicated by arrows 20, to maintain the target separations.
In current systems, a human operator steers the reference streamer by sending lateral steering commands to the lateral position controllers to drive the reference streamer to adjust feather. The other streamers are then automatically steered toward the selected separations referenced directly or indirectly from the reference streamer. But the manual positioning of the reference streamer is time-consuming and, in turns, can be hectic as well.